The invention relates to a display device having a colour display tube and a display screen with a tube neck located opposite said screen, and a convergence correction device comprising an arrangement of correction coils around the tube neck for generating correction fields.
U.S. Pat. No. 4,027,219 describes a device in which eight or twelve coils (solenoids) wound on cores of a ferromagnetic material are arranged in a row around the tube in such a way that their axes are coplanar, while they are incorporated in a circuit having controllable current sources in such a way that, upon energization, two four-pole fields and two six-pole fields are generated whose intensity and polarity are controllable for obtaining (static) convergence of the coplanar (red, green and blue) electron beams.
Drawbacks of the use of such a configuration of solenoids are:
the insensitivity, requiring a convergence circuit with relatively expensive amplifiers; PA1 the little freedom of design as regards the exact field shape; PA1 the complicated electric circuit which is required to generate all desired multipolar fields with a limited number of coils, and PA1 the limited suitability for dynamic convergence due to the large self-inductance of the solenoids.